Background: Chronic mountain sickness (CMS) is a high-altitude maladaptation syndrome that affects between 5-10 % of the world’s estimated 140 million highlanders who permanently reside >2,500 meters above sea-level (Leon-Velarde et al. 2005). It is characterised by excessive erythrocytosis, severe hypoxaemia and elevated systemic oxidative-nitrosative stress (OXNOS) subsequent to a free radical-mediated reduction in vascular nitric oxide (NO) bioavailability (Bailey et al. 2013; Bailey et al. 2019). To better define underlying molecular mechanisms, dietary risk factors and vascular consequences, we compared healthy male lowlanders (80 m, n = 10) against age/sex-matched highlanders born and bred in La Paz, Bolivia (3,600 m) with (CMS+, n = 10) and without (CMS-, n = 10) clinically diagnosed CMS.

Methods: Ethical approval protocol was obtained from the Institutional Review Boards for Human Investigation at the University of San Andres, La Paz, Bolivia (CNB #52/04), Universidad Peruana Cayetano Heredia, Lima, Péru (#101686),  University of Lausanne, Lausanne, Switzerland (#89/06, #94/10), and University of South Wales, Glamorgan, UK (#4/07), prior to registration in a clinical trials database (NCT01182792). Cephalic venous blood was assayed for systemic OXNOS using electron paramagnetic resonance spectroscopy and reductive ozone-based chemiluminescence (Bailey et al. 2019). Nutritional intake was assessed via dietary recall. Systemic vascular function and structure were assessed via flow-mediated dilatation, aortic pulse wave velocity and carotid intima-media thickness using duplex ultrasound and applanation tonometry. Following confirmation of distribution normality (Shapiro-Wilk W tests), data were analysed using one-way ANOVAs with post-hoc Bonferroni-corrected independent samples t-tests. Relationships between variables were assessed using Pearson Product Moment Correlations.

Results: Basal systemic OXNOS was permanently elevated in highlanders (P = <0.001 vs. lowlanders) and further exaggerated in CMS+, reflected by increased hydroxyl radical spin adduct formation (P = <0.001 vs. CMS-) subsequent to liberation of free ‘catalytic’ iron consistent with a Fenton and/or nucleophilic addition mechanism(s). This was accompanied by elevated global protein carbonylation (P = 0.046 vs. CMS-) and corresponding reduction in plasma nitrite (P = <0.001 vs. lowlanders). Dietary intake of vitamins C and E, carotene, magnesium and retinol were lower in highlanders and especially deficient in CMS+ due to reduced consumption of fruit and vegetables (P = <0.001 to 0.028 vs. lowlanders/CMS-). Systemic vascular function and structure were also impaired in highlanders (P = <0.001 to 0.040 vs. lowlanders) with more marked dysfunction observed in CMS+ (P = 0.035 to 0.043 vs. CMS-) in direct proportion to systemic OXNOS (r = -0.692 to 0.595, P = <0.001 to 0.045).

Conclusions: Collectively, these findings suggest that lifelong exposure to iron-catalysed systemic OXNOS, compounded by a dietary deficiency of antioxidant micronutrients, likely contributes to the systemic vasculopathic complications and increased morbidity/mortality in CMS+.